Role of conserved TGDGVND-loop in Mg2+ binding, phosphorylation, and energy transfer in Na,K-ATPase

In the P-domain, the 369-DKTGTLT and the 709-GDGVNDSPALKK segment are highl y conserved during evolution of P-type E-1-E-2-ATPase pumps irrespective of their cation specificities. The focus of this article is on evaluation of the role of the amino acid residues in the P domain of the ce subunit of Na ,K-ATPase for the E1P[3Na] --> E2P[2Na] conversion, the K+-activated dephos phorylation, and the transmission of these changes to and from the cation b inding sites. Mutations of residues in the TGDGVND loop show that Asp(710) is essential, and Asn(713) is important, for Mg2+ binding and formation of the high-energy MgE1P[3Na] intermediate. In contrast Asp(710) and Asp(713) do not contribute to Mg2+ binding in the E2P-ouabain complex. Transition to E2P thus involves a shift of Mg2+ coordination away from Asp(710) and Asn( 713) and the two residues become more important for K+-activated hydrolysis of the acyl phosphate bond at Asp(369). Transmission of structural changes between the P-domain and cation sites in the membrane domain is evaluated in light of the protein structure, and the information from proteolytic or metal-catalyzed cleavage and mutagenesis studies.